Comparison of irreversible and reversible cluster formation

Covalent and reversible cluster molecules were synthesized by an A₃B₂ type gelation. Crosslinking of three-arm hydroxyl-terminated star polymers with 2,4-toluenediisocyanate gave branched polymers, while the reversible analogue was made by crosslinking of tertiary amine-terminated star polymers with bis(4-hydroxy-3,5-dinitrophenyl) adipate. Gelation process was followed by static and dynamic light scattering. The extent of reacted groups was measured with UV spectroscopy. Growth of the covalent clusters could be described in terms of percolation scaling laws. The experimental gel point (P_{OH, cr} = 0.70) was shifted significantly from the theoretical predicted gel point (P_{OH, cr} = 0.50), indicating extensive ring formation during the gelation. The reversible endlinking reaction gave no macroscopic gelation, though increase of the cluster dimensions was observed. Ring formation proved to be an important side reaction in both cases; however, the ring formation ability seems to change in a different manner during the course of a gelation.     

Spin-transition in nearly cubic site in [FeII(L)3][PF6]2

The spin-transition (¹A₁?⁵T₂) behaviour of a new mononuclear iron(II) compound [Fe^II(L)₃][PF₆]₂[L = 2-[3-(2′-pyridyl)pyrazole-1-ylmethyl]pyridine] has been investigated by ⁵⁷Fe Mössbauer spectroscopy. Analysis of the Mössbauer spectra revealed low value of the quadrupole splitting of the high-spin state which reflects iron(II) to be in nearly cubic lattice site. Mössbauer spectra under light show the light-induced excited spin state trapping effect and the observed quadrupole splitting of the metastable high-spin state is found little sensitive to the high-spin fraction value. DFT calculations are in progress to document the almost cubic nature of the ligand-field acting on the iron atom.     

An expedient reductive method for conversion of ketoximes to the corresponding carbonyl compounds

A wide array of readily prepared pivalates of ketoximes can be converted to the corresponding ketones in good yields by treatment with iron powder in THF containing catalytic amounts of both trimethylsilyl chloride and glacial acetic acid at room temperature for 30 min, followed by a brief aqueous workup.     

The Least-Perimeter Partition of a Sphere into Four Equal Areas

We prove that the least-perimeter partition of the sphere into four regions of equal area is a tetrahedral partition.     

为在巴西引入数字电视提交的"公开咨询报告"文稿第291号

1 引言 由"国家电信局"(National Telecommunications Agency,Anatel)主持的数字电视在巴西的首次"公开咨询报告"(Public Consultation)的一年后,我们注意到:在这种技术的领域内,已经出现很多不同的显著事实.除了"CPqD基金会"报告以外,新的对比性测试曾经在美国(ATSC对DVB-T)和中国香港(ATSC,DVB-T和ISDB-T)都进行过.还有一些报道表明:中国台湾也进行了对比性测试,目的是确立自己的数字电视标准.     

Solvent-free fluorination of partially-chlorinated heterocyclics: Synthesis of 2,6-difluoropyridine from 2,6-dichloropyridine

Previous studies on the solvent-free fluorination of partially-chlorinated heterocyclics such as 2-chloropyridine, 2,3,4,6- and 2,3,5,6-tetrachloropyridine, and 2,4,6-trichloropyrimidine with alkali metal fluorides or hifluorides revealed an inconsistent product pattern: degradation, partial or complete halogen exchange could occur. The present study concerned application of the solvent-free halogen exchange technique (potassium fluoride) to 2,6-dichloropyridine (I). The latter, as well as its fluorination products, exhibited good stability at 400° to provide an 80% yield of 2,6-difluoropyridine (III). Moderation of fluorination conditions also permitted isolation of the precursor, 2-chloro-6-fluoropyridine (II), a new compound. The above solvent-free process to III is superior to the previously-reported halogen-exchange route in dimethyl sulfone solvent on the basis of yield, reaction time and number of processing steps.